Optical resonators for lasers have typically used optical resonators consisting of a pair of spaced reflecting surfaces which reflect light rays back and forth through a gain region where laser pumping takes place. Energy is extracted from the resonator around the outside edge of one of the mirrors or by providing an opening in one of the mirrors or by half silvering one of the mirror surfaces so that a portion of the light rays are transmitted through or around the mirror while the remaining portion is reflected back through the gain region. In some types of chemical lasers it is desirable to provide a resonator in which the light rays pass through an annular cylindrical shaped gain region. This has been accomplished in the past by means of a conical mirror unit positioned between the two reflecting surfaces of the resonator. The conical mirror unit, sometimes referred to as a W-axicon, consists of a central 90.degree. conical mirror surrounded by a ring mirror having a 90.degree. conical surface. Light directed in a compact beam along the axis of the central conical mirror is reflected radially outwardly at 90.degree. to the axis of the cone where it impinges on the conical reflecting surface of the surrounding ring mirror. The conical surface of the ring mirror in turn directs the light rays in a direction parallel to the axis of the cone within an annular cylindrical volume concentric with the axis of the cones. A flat mirror may be used to reflect the light in the cylindrical volume back to W-axicon for concentrating it again into a compact beam.
Such a mirror arrangement provides the desired annular shaped gain region but presents formidable alignment problems of the reflecting surfaces and also presents problems in extracting high levels of optical energy from the resonator.
An improved annular resonator used to extract optical power from the annular gain region was developed utilizing four mirror assemblies. This resonator, shown in FIG. 1, consists of a feedback mirror having a substantially spherical surface contour, a W-axicon consisting of a central conical mirror surface and a surrounding ring having a conical mirror surface, a second or rear annular ring having a conical reflector surface, and a scraper mirror having a flat surface with a central hole positioned between the feedback mirror and the central conical mirror. The gain region from which optical power is extracted is contained within an annular volume extending between the two ring mirrors or axicons. The second ring mirror acts as a retroreflector that causes each ray to return on the opposite side of the annular region, allowing for compensation of slight misalignments. In operation, a spherical wave beginning at the feedback mirror, passes through the opening in the scraper mirror and impinges on the central conical mirror of the W-axicon assembly. This incident compact spherical beam is reflected by the central conical mirror radially outwardly to the first ring mirror from which it is reflected toward the rear ring mirror as an annular shaped beam. This annular beam extending between the two ring mirrors fills the annular gain region where power is extracted by well-known lasing action. The rear ring mirror rotates the beam through 180.degree. in two reflecting stages and directs a nominally collimated annular beam back to the first ring. The central conical mirror then directs the light rays in a nominally collimated beam back to the scraper mirror. The portion of the collimated beam that passes through the hole in the scraper is again reflected by the back mirror and provides feedback for another pass around the closed path of the resonator.
While the resonator configuration of FIG. 1 has the advantage that only two conical mirror assemblies are required which can be diamond turned after assembly to provide permanent and accurate alignment, this resonator design has several problems when used for practical applications. To insure that the resonator operates in a single tranverse uniphase mode, the central conical mirror must be accurately finished right out to the very tip of the central cone. Also the azimuthal figure error permitted on the conical mirrors is extremely small, being a small fraction of the laser wavelength. Also in transforming the collimated annular beam into the collimated compact beam directed at the scraper mirror, very high energy levels near the tip of the cone can occur which can damage the tip region of the central conical mirror.